Various implantable therapeutical devices, such as drug delivery, gene therapy, and cell encapsulation devices, have been disclosed over the years. A common feature of most of these devices is the use of selectively permeable, or semi-permeable, membranes to construct all or part of the device. These membranes contain their respective therapeutic agents and delivery systems within the particular device while being permeable to the desired therapeutical product. For cell encapsulation devices, the membranes are also permeable to life sustaining substances and to cellular waste products.
When implanted in a recipient, the typical biological response by the recipient to most of these therapeutical devices is the formation of a fibrotic capsule around the device. With most drug delivery and gene therapy devices, this can limit the performance of the device, particularly when the therapeutic agent has a short half-life. For cell encapsulation devices, a fibrotic capsule encasing the device most often deprives the encapsulated cells of life sustaining exchange of nutrients and waste products with tissues of a recipient. The result is usually fatal to the encapsulated cells. Furthermore, a fibrotic capsule encasing a therapeutical device usually makes surgical retrieval of the device difficult.
When certain therapeutical devices are implanted in a recipient, predominantly vascular tissues of the recipient can be stimulated to grow into direct, or near direct, contact with the device. On one hand, this is desirable because the therapeutical product of the device can then be delivered directly to the circulation of the recipient through the vascular tissues that are in contact with the device. On the other hand, this is undesirable because once vascular tissues of a recipient have grown in contact with one of these implantable therapeutical devices, removal of the device requires surgical dissection of the tissues to expose and remove the device. Surgical dissection of vascular tissues, particularly capillary tissue, can often be a difficult and painful procedure. Whether encased in a fibrotic capsule or surrounded with vascular tissue, the problem of retrieving these implanted devices is a considerable drawback of the devices.
For cell encapsulation devices, an alternative to retrieving and replacing the entire device in a recipient is to retrieve and replace the cells contained in the device. U.S. Pat. No. 5,387,237, issued to Fournier et al., is a representative example of a cell encapsulation device that has at least one opening into the device through which cells can be introduced and removed. Cells are introduced and removed in this, and other similar devices, as a suspension or slurry. Since most cell encapsulation devices are intended to correct a metabolite deficiency in a recipient caused by dysfunction or failure of certain of the recipient's cells, tissues, or organs, the source of the replacement cells is rarely the recipient. In a situation where non-autologous cells are used in this type of cell encapsulation device, the problem of contaminating a recipient with the foreign cells during loading, removal, or refilling of the device is ever present. One solution to this contamination problem would be to enclose the cells in a container that can be placed, removed, and replaced in a device as a unit.
A retrievable cell encapsulation envelope enclosed in an implantable permselective membrane for use as an artificial endocrine gland is disclosed in U.S. Pat. No. 4,378,016 issued to Loeb. The Loeb device comprises a housing made of an impermeable hollow stem and a permselective membrane sack. The hollow stem has a distal end defining an extracorporeal segment, a percutaneous segment in the mid-region, and a proximal end defining a subcutaneous segment. The sack is adapted to receive an envelope containing hormone-producing cells and has an access opening that is coupled to the proximal end of the hollow stem. In a preferred embodiment, the cell containing envelope is in the form of a flexible collar. The flexible collar is partially collapsible to allow for easier placement and replacement of the envelope in the sack. Once in place, the flexible collar also provides a snug fit between the envelope and the sack. Placement and replacement of a cell containing envelope in the sack portion is accomplished manually with forceps, or the like. Retrieval of the envelope from the sack can be aided with a guidewire attached to the envelope. In one embodiment of the Loeb device, the sack has openings at both ends that are implanted percutaneously. In this embodiment, the cell containing envelope may be inserted or removed through either end of the device.
The housing of the Loeb device is surgically implanted in a recipient through the abdominal wall so the distal end of the stem protrudes from the recipient, the proximal end of the stem resides subcutaneously with respect to the abdominal wall, and the sack portion is placed in the peritoneal cavity surrounded in peritoneal fluid. According to Loeb, the sack allows hormones, nutrients, oxygen, and waste products to flow in and out of the sack while preventing bacteria from entering the patient. The sack and the envelope are said by Loeb to be permeable to nutrients and hormones, but impermeable to the hormone-producing cells and immune response bodies. Upon implantation of the device in a patient, the cells contained therein are said to take over the function of the corresponding natural gland, sense the amount of hormone needed, and produce the correct amount of the desired hormone.
Implanted cell encapsulation devices, particularly those intended as an artificial endocrine gland, usually require a high rate of flux of nutrients and waste products between the encapsulated cells in the device and tissues of the recipient. Having a cell encapsulation device in close, or direct, association with a vascular structure usually provides the highest rate of nutrient and waste product flux for such a device. Loeb does not teach the value of vascularization of the sack portion of the housing, however. Nor is the Loeb device implanted in a part of the body that is particularly vascularized.
Brauker et al. disclose a cell encapsulation device in U.S. Pat. No. 5,314,471 that requires close association of host vascular structures with the device. According to Brauker et al., "conventional implant assemblies and methodologies usually fail to keep the implanted cells alive long enough to provide the intended therapeutic benefit." Cell death in these implanted devices is said by Brauker et al. to be due in large part to an ischemia imposed on the cells during the first two weeks following implantation. Brauker et al. conclude that "the cells die because conventional implant assemblies and methodologies themselves lack the innate capacity to support the implanted cells' ongoing life processes during the critical ischemic period, when the host's vascular structures are not nearby." Brauker et al. state that in order for implanted cells to survive and function on a long term basis, the host must grow new vascular structures in association with the device. Brauker et al. note that a host will not naturally provide new vascular structures to an implanted cell encapsulation device. According to Brauker et al., the host must be stimulated by the implant assembly itself to grow new vascular structures close to the cell encapsulation device. Angiogenic stimulus can be provided by angiogenic factors applied to the cell boundary of the Brauker et al. device or by certain cell types encapsulated in the device. Growth of vascular tissue in association with a device of Brauker et al. will make removal of the device from a host difficult, however.
An implantable containment apparatus made of selectively permeable polymeric material that permits a therapeutical device, such as a drug delivery, a gene therapy, or a cell encapsulation device, to be placed and replaced in a recipient without damaging or disturbing tissues associated with the selectively permeable polymeric material would be useful. Such an apparatus that becomes closely associated with vascular structures without the need to supply angiogenic factors to induce the close vascularization would also be useful. A method of easily placing and replacing a therapeutical device in an implantable containment apparatus of the present invention would be further useful.